Parsing tests: split LinksTest into per-ParserMode files

Replaces the monolithic LinksTest with one test class per ParserMode:
CamelcaselinkTest, EmaillinkTest, ExternallinkTest, FilelinkTest,
Inter

Parsing tests: split LinksTest into per-ParserMode files

Replaces the monolithic LinksTest with one test class per ParserMode:
CamelcaselinkTest, EmaillinkTest, ExternallinkTest, FilelinkTest,
InternallinkTest, WindowssharelinkTest. Media-parser dispatch tests
move into the existing MediaTest alongside the audio/video coverage.

ExternallinkTest also adds unit coverage for the GFM autolink
extension trim step: balanced parens, trailing entity refs (valid
named, numeric, and unknown round-trip), non-trailing entities staying
inside the URL, and mixed paren/entity peeling.

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